1. Field of the Invention
The present invention relates to a method for determining a periodic structure or pattern in a line spectrum. More particularly, the present invention relates to a method for determining a periodic structure or pattern in a line spectrum caused by a modulation having a fundamental period, such as a periodically moving part as detected by a coherent Doppler radar.
2. Description of the Related Art
In many technical fields it is necessary to determine a periodic structure or pattern from an amplitude spectrum. Periodically moving or rotating parts, for example, helicopter rotor blades, and/or the rotational movement of a turbine of an airplane, can be inferred from periodic patterns in, for example, coherent Doppler radar spectra. Additional information of this type is necessary in, for example, target classification of radar echoes by a ground radar station of an airport.
Periodic structures, or patterns, also occur in machinery during a so-called noise analysis of the machinery. For example, the state of wear of a cutting tool can be inferred from noise, whether airborne or structure-borne noise, produced by a metal-cutting machine, e.g., a lathe or milling machine, through spectral analysis of the noise. A periodic pattern in the noise caused by rotational motion of the lathe or milling machine produces an amplitude spectrum which can be analyzed.
Determination of periodic patterns is also required in numerous areas of electrical technology such as, for example, in monitoring and/or identifying a radio transmitter. In applications of this type, a spectrum analyzer can detect an amplitude spectrum. Generally, this detected spectrum includes a reference line, also referred to as a zero line or base line, and a plurality of line spectra which are contiguous on one or both sides of the zero line. In such a line spectrum, a periodic pattern caused by, for example, a periodic amplitude modulation, can be determined because the harmonics of the modulation, also known as continuation lines, are present for every spectral line contiguous to the zero line.
An evaluation of this type is difficult or even, perhaps, impossible when the periodic pattern which is to be analyzed is covered or masked by a number of interfering spectral lines caused by, for example, noise and/or non-periodic, deterministic spectral components.